In many communications systems, terminals which may be used for different purposes, such as for transmitting voice, video, fax, multimedia, information, text, program and/or measurement data, are increasingly being connected without the use of wires. A connection to such mobile terminals is normally produced via so-called base stations which are connected to a communications network and can be connected to the mobile terminals via an air interface. In the following text, the expression mobile terminals should be understood also as meaning so-called cordless terminals.
User data is generally interchanged via the air interface between a mobile terminal and a base station within time frames which are predetermined by a clock, and which are referred to in the following text as radio time frames.
The area around a base station in which a wire-free connection of predetermined quality can be set up between a mobile terminal and this base station is also referred to as the radio cell of this base station. In order to supply a larger area with connection capabilities, a number of base stations are generally distributed over the area to be supplied, such that their radio cells form a radio network covering the entire area. A mobile terminal which is registered in such a radio network can, in this case, move in any desired way between each of the base stations which are located within radio range in this radio network. The process of a mobile terminal being passed on from a first base station to a second base station while a connection exists is also referred to as a handover. In general, such a change in the connection profile should take place as far as possible without any perceptible interruption in the connection. This is also referred to as a seamless handover.
However, to carry out a seamless handover, the base stations involved must be synchronized to one another with respect to the air interface. For example, user data to be transmitted via a DECT air interface is embedded in radio time frames whose starts in the base stations involved in a seamless handover must not differ from one another by more than 2 μs.
In this context, the expression synchronization of base stations should be understood as meaning, in particular, synchronization of radio time frames, on which a user data interchange with mobile terminals is based, from different base stations.
Laid-open Specification WO 96/38990 discloses a mobile communications system, in which base stations are each connected to a private branch exchange via an S0 interface in accordance with the ISDN Standard. In this case, a reference clock is transmitted from the private branch exchange to the base stations via the S0 interface on the physical layer of the transmission protocol that is being used. The clock generators in these base stations are synchronized on the basis of the reference clock, which is received in the same way by all the base stations.
With regard to the increasing networking of communications systems, increasing integration of voice and data services, and increasing use of complex service features by mobile terminals, it is being found that the connection of base stations via S0 interfaces is too inflexible. The lack of flexibility is a result, in particular, of the transmission of the reference clock in the physical layer of the transmission protocol that is being used, since continuous layer 1 connections between the private branch exchange system and the base stations are required for this purpose.
An object of the present invention is to specify a method and a system which is more flexible than the prior art for synchronization of base stations in a mobile communications network, in particular for the purpose of a seamless handover.